This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
A conventional gas turbine engine often has a turbine compressor that is mechanically linked to a turbine expander through a shaft. The turbine compressor can be used to compress a flow of air ingested by the turbine compressor. The compressed air is then passed to a combustor. In the combustor, fuel is injected and ignited to create a continuous flame. The high pressure exhaust gases from the flame are flowed into the turbine expander, which generates mechanical energy from the exhaust gas as it expands. The mechanical energy, transferred through the shaft to the turbine compressor, is used to power the compression of the air. Additional mechanical energy is produced, over the amount used to compress the ingested air, and harvested for other purposes, for example, to generate electricity. The flame temperature can exceed the metallurgical limits of the combustor can, so an excess amount of air is often used to provide cooling. However, this arrangement may create a higher amount of pollutants, such as nitrogen oxides (NOxs).
Capturing carbon dioxide from the exhaust gas for other uses may be problematic for a number of reasons. For example, there is a low concentration of carbon dioxide in the exhaust of a conventional gas turbine and a very large volume of gas has to be treated. The exhaust stream is relatively low pressure, e.g., around 1050 kPa. The exhaust stream may be very high temperature at around 425° C. to 700° C. Further, the exhaust gas may contain a large amount of oxygen that may interfere with CO2 extraction or use. Finally the exhaust gas may be saturated with water from cooling, which can increase a reboiler duty in the CO2 extraction system.
The combustion of fuel within a combustor, e.g., integrated with a gas turbine, can be controlled by monitoring the temperature of the exhaust gas leaving the expander, because temperatures are generally too high in the combustor for existing instrumentation. At full load, typical gas turbines adjust the amount of fuel introduced to a number of combustors in order to reach a desired combustion gas or exhaust gas temperature.
However, controlling the amount of oxidant introduced to the combustor can be desirable when an objective is to capture carbon dioxide (CO2) from the exhaust gas. Current carbon dioxide capture technology is expensive for several reasons. For example, the low pressure and low concentration of carbon dioxide in an exhaust gas. The carbon dioxide concentration, however, can be significantly increased from about 4% to greater than 10% by operating the combustion process under substantially stoichiometric conditions. Further, a portion of the exhaust gas may be recycled to the combustor as a diluent for cooling the products of combustion instead of air. The benefit of using recycle gas as the coolant is that the amount of oxygen in the recycle gas sent to the CO2 capture facilities can be controlled at low levels. A low oxygen level may allow a wide range of solvents to be utilized for the capture of carbon dioxide.
The enhanced exhaust gases may be captured for use by other systems, for example, directly from the exhaust of the gas turbine. However, if a gas turbine is being supplied an oxidant from a separate source, it may be more effective to compress the exhaust in the turbine compressor of the gas turbine, and recycle the compressed gas to the combustors as a coolant, then capture a high pressure bleed flow during the control of the recycle flow. Numerous studies have examined the concept of recycling a portion of the exhaust gases to the combustor.
For example, U.S. Pat. No. 4,271,664 to Earnest discloses a turbine engine with exhaust gas recirculation. The engine has a main power turbine operating on an open-loop Brayton cycle. The air supply to the main power turbine is furnished by a compressor independently driven by the turbine of a closed-loop Rankine cycle which derives heat energy from the exhaust of the Brayton turbine. A portion of the exhaust gas is recirculated into the compressor inlet during part-load operation. However, the recycled exhaust gas is taken from a final vent, without further compression. Further, no additional uses are disclosed for the recycled exhaust.
U.S. Patent Application Publication No. 2009/0064653 by Hagen, et al., discloses partial load combustion cycles. The part load method controls delivery of diluent fluid, fuel fluid, and oxidant fluid in thermodynamic cycles using a diluent to increase the turbine inlet temperature and thermal efficiency in part load operation above that obtained by relevant art part load operation of Brayton cycles, fogged Brayton cycles, or cycles operating with some steam delivery, or with maximum steam delivery.
International Patent Application Publication No. WO/2010/044958 by Mittricker, et al., discloses methods and systems for controlling the products of combustion. One embodiment includes a combustion control system having an oxygenation stream substantially comprising oxygen and carbon dioxide and having an oxygen to carbon dioxide ratio, then mixing the oxygenation stream with a combustion fuel stream and combusting in a combustor to generate a combustion products stream having a temperature and a composition detected by a temperature sensor and an oxygen analyzer, respectively. The data from the sensors are used to control the flow and composition of the oxygenation and combustion fuel streams. The system may also include a gas turbine with an expander and having a load and a load controller in a feedback arrangement.
International Patent Application Publication No. WO/2009/120779 by Mittricker, et al., discloses systems and methods for low emission power generation and hydrocarbon recovery. One system includes integrated pressure maintenance and miscible flood systems with low emission power generation. An alternative system provides for low emission power generation, carbon sequestration, enhanced oil recovery (EOR), or carbon dioxide sales using a hot gas expander and external combustor. Another alternative system provides for low emission power generation using a gas power turbine to compress air in the inlet compressor and generate power using hot carbon dioxide laden gas in the expander.
The prior systems disclose adding the diluent to the oxidant prior to or during the combustion process. Further, conventional gas turbine systems and the systems disclosed above, may obtain a high pressure stream from a bleed valve on the compressor for other purpose, such as heating the inlet air. This bleed stream may is normally limited to 5 to 10% of the total flow from the compressor. If the oxidant being used in the stoichiometric combustion of the fuel is air, very large extraction rates (about 40% of the total recycle gas stream) are required. These large extraction flows would not be possible on commercially available gas turbines without expense modifications.